The effect of additional day-length produced by supplemental illumination (30 foot-candles, 5 to 24 hrs. daily) during the winter months upon 100 spp. of plants grown in the Univ. of Colorado greenhouse is reported. In practically all cases the experimental plants were taller, somewhat paler, less sturdy, and had a poorer root system than the controls. The time from planting to blooming was shortened in 60 spp., not greatly affected in 28 spp., lengthened in 12 spp. Of the plants hastened in blooming by the artificial light most are natives of the Temperate Zone; those which show no difference from the controls are largely tropical. A few perennials were brought into blossom the first season through the influence of supplemental light.